1. Field of the Invention
This invention relates to an automatic power control circuit, such as for use with a radio telecommunication apparatus. More specifically, this invention relates to an automatic power control circuit designed to stabilize a transmission output of a radio telecommunication apparatus.
2. Description of the Prior Art
Automatic power control (APC) circuits have been used with such radio telecommunication devices as cellular mobile telephone systems and the like. With particular reference to FIG. 1, the structure and operation of a conventional APC circuit can be described.
An APC circuit is shown generally at 21. Typically, APC circuitry is used in conjunction with peripheral circuit elements, such as reference signal generating circuit 22, band pass filter 23 and antenna 24.
APC circuit 21 consists of voltage detection circuit 25, comparator 26 and power amplifier 27. As illustrated in FIG. 1, an input signal is input to power amplifier 27, where the signal is amplified and supplied to both band pass filter 23 and voltage detection circuit 25.
Voltage detection circuit 25 then rectifies the signal and detects its output voltage. The detected voltage is input to comparator 26. Additionally, a reference signal, by which the transmission output of power amplifier 27 is determined, is also input to comparator 26 from reference signal generating circuit 22. At comparator 26, the output voltage from detection circuit 25 and the reference signal from reference signal generating circuit 22 are compared. If the result of the comparison indicates that the output voltage is not at the same level as that of the reference signal, the power output from power amplifier 27 is adjusted so that the level of the output voltage detected in the detection circuit 25 is equalized with that of the reference signal.
Using such APC circuit, a signal having a constant output level is input to the band pass filter. Thus, unnecessary or spurious signals, such as harmonic distortion, can be removed from the input signal before being output from antenna 24.
FIG. 2 shows the electrical characteristics of band pass filter 23. In the case of an automobile phone system, for example, 832 channels are assigned at respective frequencies in the band defined between f.sub.L (824 MHz) and f.sub.H (849 MHz), with center channel f.sub.C in the middle of the band (836.5 MHz). When the reference signal for the transmission output from the APC circuit is set in accordance with the center channel f.sub.C, loss of the transmission output, due to the band pass filter's frequency characteristics, is very slight for channel frequencies near fC. However, at channel frequencies approaching fL and fH, output deviation A (loss of output) between the channels increases. As a result, actual output will deviate from channel to channel.
The prior art APC circuitry detects the transmission output immediately after power amplification and immediately before filtering. Thus, the transmission output will inevitably deviate from one channel to another, as described above, due to the influence of the frequency characteristics of the band pass filter. This has been a major disadvantage of conventional prior art APC circuits.